1. Field of the Invention
The resent invention relates to a hydraulic control system, and, more particularly, to a hydraulic control system, having a three-way solenoid valve, for an automatic transmission.
2. Description of Related Art
Typically, automatic transmissions for automobiles have torque converters and transmission gear mechanisms. Each such torque converter and transmission gear mechanism includes a plurality of frictional coupling elements which are selectively locked or coupled and are unlocked or released. Such frictional coupling elements may include clutches and brakes and are locked and unlocked so as to place the automatic transmission into desired transmission modes or ranges and gears according to driving conditions. Selectively locking and unlocking of these frictional coupling elements is performed by the use of actuators associated with the respective frictional coupling elements which are controlled by means of a hydraulic control circuit.
Hydraulic control circuits of this kind typically include various hydraulic control valves, such as, for instance, a line pressure regulator valve, a manual valve which is operated manually to change from one transmission range to another, a plurality of shift valves for selectively activating the actuators so as to provide any desired gear. In order to provide for the regulator valve optimum regulation of line pressure according to driving conditions including selected gears, some hydraulic control circuits of this kind are provided with duty solenoid valves which causes a regulator valve to turn ON and OFF periodically. If such a hydraulic control circuit includes a two-way type of duty solenoid valve which opens and closes its drain port, the duty solenoid valve is connected at the drain port to a pressure line between an orifice and a subject frictional coupling element. The duty solenoid valve is operated at a duty rate (a rate of an opened period of time relative to a period of time for one cycle of NO-OFF operation) necessary to develop a target pressure determined according to engine output.
However, as well known in the art, viscosity of working oil used in the hydraulic control circuit changes greatly due to a change in oil temperature. Specifically, the working oil tends to provide an increase in viscosity with a decrease in oil temperature. Consequently, when the duty solenoid valve is used to regulate oil pressure for operating the frictional coupling element, the frictional coupling element tends to change its operative characteristics. For example, the working oil suffers increased resistance at low temperatures, causing the duty solenoid valve to provide controlled line pressure P lower under relatively high oil temperatures than under relatively low oil temperatures.
In order for a hydraulic control circuit having the duty solenoid valve to avoid such a problem, as described in Japanese Patent Publication No. 5-17430, the duty rate may be changed so as to shorten the rate of opened period of time relative to one cycle at low oil temperatures.
With the prior art hydraulic control circuit, even when the resistance of working oil against the orifice is increased due to a drop in oil temperature, the working oil discharged from the duty solenoid valve is less decreased in amount, avoiding an excessive drop in oil pressure at low oil temperatures.
Some hydraulic control circuits of this kind include three-way type of duty solenoid valves. While such a hydraulic control circuit may have advantages over the prior art, nevertheless, a specific restraint must be imposed upon the duty rate. For the purpose of providing a brief background that will enhance an understanding of the operation of the present invention, reference is made to FIGS. 12 and 13.
Referring to FIG. 12, a three-way type of duty solenoid valve E1 has a plunger E2 disposed for axial movement in a valve housing E3. This plunger E2 with a valve head E2a moves in the axial direction so as to bring the valve head E2a into close contact selectively with valve seats E4a and E4b formed axially separated in the valve housing E3. With this selective contact of the valve head E2a with these valve seats E4a and E4b, an output port E5a, which is in communication with a related frictional coupling element, is brought into communication selectively with an input port E5b through which line pressure is introduced into the valve housing E3 and a drain port E5c. The plunger E2 is always urged by means of a coil spring E6 so as to hold the valve head E2a in close contact with the valve seat E4b, thereby keeping the input port E5b in communication with the output port E5a and disconnecting the communication of the output port E5a with the drain port E5c. A solenoid coil E7, is mounted on the valve housing E3 so as to surround the plunger stem E2b.
When the solenoid coil E7 is energized, it forces the plunger E2 against the coil spring E6 so as to bring the valve head E2a into close contact with the valve seat E4a and thereby to keep the output port E5a in communication with the drain port E5c. As long as the solenoid coil E7 is continuously energized, the plunger E2 is urged against the coil spring E6 so as to hold the valve head E2a in close contact with the valve seat E4a, thereby keeping the output port E5a in communication with the drain port E5c and disconnecting the communication of the output port E5a with the input port E5b. Periodically energizing and deenergizing the solenoid coil E7 at a duty rate causes the solenoid valve changes an input oil pressure at the input port E5b as a regulated output oil pressure at the output port E5a according to the duty rate.
As shown by a solid line in FIG. 13, under high oil temperatures at which the working oil has relatively high viscosity, the solenoid valve provides a regulated or controlled line pressure P which decreases at a substantially constant incline with an increase in duty rate after an insensitive range of duty rates. On the other hand, when the duty rate is low so as thereby to disconnect the communication of the drain port E5c with the output port E5a for a relatively long period of time under low oil temperatures at which the working oil has relatively low viscosity, the solenoid valve causes a reduction of discharged oil in amount through the drain port E5c, providing the controlled line pressure P at the output port E5c increased as compared with that under high oil temperatures.
Conversely, when the duty rate is high so as thereby to disconnect the communication of the drain port E5c with the output port E5a for a relatively short period of time under low oil temperatures, the solenoid valve causes a reduction of introduced oil in amount through the input port E5b, providing the controlled line pressure P at the output port E5c decreased as compared with that under high oil temperatures.
As shown by a dotted line in FIG. 13, in a range of relatively low oil temperatures, the controlled line pressure P is changed relatively lower in a range of duty rates D, greater than a predetermined specific duty rate Da, than in a range of relatively high oil temperatures. For example, if controlled line pressure of a level Pa is given for a duty rate Db less than the specific duty rate Da at higher oil temperatures, the controlled line pressure P changes higher from the level Pa to a level Pb for the same duty rate Db. On the other hand, if line pressure of a level Pc is given for a duty rate Dc greater than the specific duty rate Da at higher oil temperatures, the controlled line pressure P changes lower from the level Pc to a level Pd for the same duty rate Db.
Consequently, the level of controlled line pressure necessary to activate the related frictional coupling element is relatively higher as compared with the level of desired line pressure, causing the frictional coupling element to produce shocks during coupling or relatively lower as compared with the level of desired line pressure, prolonging a time necessary for the automatic transmission to perform shift operation.